Common floorcovering adhesives intended for use with carpet or vinyl floor coverings are water-based emulsions based upon either acrylic or Styrene Butadiene (SBR) latex. Acrylic-based formulas, because of expense and low bond strength, are generally used only when necessary for a pressure-sensitive application or for resistance to plasticizers in pure vinyl backings. SBR-based floorcovering adhesives are by far the predominant type and require the addition of a tackifier for development of bond strength and proper modulus. The traditional means of tackifying an adhesive of this type is the addition of a hydrocarbon resin. Since these resins are solid at room temperature, they must be liquefied in order to be incorporated into an emulsion (consisting of petroleum process oil, resin, surfactant, and water). This is usually done by dissolving the resin in an organic solvent carrier. Organic solvents are heavily scrutinized by government agencies for their contribution to air pollution, their flammability (in some cases), and they also degrade adhesive properties by slowing down curing time, swelling and weakening the rubber component, shortening the life of the applied product, and raising the level of surfactant required for emulsion stability (which in turn degrades the adhesive further).
It is possible to melt resin to liquefy it, but keeping it liquid throughout completion of the adhesive batch poses problems. Typical resins which are compatible with SBR-based floorcovering adhesives have softening points above 212 degrees F. Resins with lower softening points are too soft at room temperature to develop the final bond strengths required. Solvent-free resin emulsions are available from suppliers to the floorcovering adhesive industry. These emulsions begin by melting the resin (done at a temperature well above the boiling point of most liquids) and slowly adding a heat-stable surfactant and then water. Because of the temperature required, this emulsification must take place at elevated pressure to avoid boiling off the water. Upon cooling, the emulsion will remain liquid and stable. The disadvantages of this process are the expense and hazards associated with high pressure equipment, high surfactant levels (slows drying time and weakens bond strength), brittle aging characteristics, and limitation of solids content in the final product (resin emulsions are limited to about 60% solids).
It is possible to dissolve resin in a solvent carrier, form an emulsion with surfactant and water, and then fractionally evaporate the solvent. This process has all of the disadvantages of the above process, with the added cost of solvent recovery equipment. This process also does not eliminate the exposure of employees to organic solvents.